1. Field of the Invention
The invention relates to an all-optical device for directly controlling a light with a light, and more particularly to an ultra-high speed all-optical device used as an optical control device in optical fiber communication and optical data processing.
2. Description of the Related Art
It is absolutely necessary to drive a device for accomplishing optical control at higher speed in order to operate an optical fiber communication system or data processing system at higher speed. In a conventional optical control device, the optical control has been carried out with electrical signals. However, in these days, attention is attracted to a process in which a light is directly controlled with a light, in order to carry out higher speed operation. This process has many advantages including that the operation speed thereof is not restricted by a CR time constant of an electrical circuit, and that optical pulses by which it is possible to generate pulses quite shorter than electrical pulses are directly available. However, many unsolved problems have to be overcome in order to establish such an all-optical device. In particular, many requirements have to be satisfied simultaneously for establishing such an all-optical device. The many requirements include functions of the device, low power operation, high transmittance of a signal light, and high repetition operation. It is preferred for the functions of the device to include that a route of a signal light can be switched by a control light, and also that such devices can be cascaded. Devices which satisfy these requirements include a Mach-Zehnder type waveguide device and a directional coupler type waveguide device. Among these two devices, a Mach-Zehnder type waveguide device is considered to be superior to a directional coupler type waveguide device because the former needs only half the optical power of the latter for operation.
Whatever form an all-optical device may have, the operation of an all-optical device is based on nonlinear refractive index change. In other words, the speed and energy for operation of an all-optical device is dependent on the speed and/or efficiency of nonlinear refractive index change. There are various nonlinear optical phenomena which accompany nonlinear refractive index change. Such phenomena can be grouped into two groups, one of which is resonantly enhanced phenomena and the other is not. Even in the present stage, if the nonresonant effects are utilized, an all-optical device could perform ultra-high repetition operation at more than T Hz. However, such an all-optical device has the disadvantage that it requires a high optical power. Accordingly, it is necessary to reduce an optical intensity by resonantly enhanced effects. The resonantly enhanced effects can be grouped into coherent effects and others.
For accomplishing an ultra-fast response, the coherent effects are preferred in which a response time is not restricted by a longitudinal relaxation time of an electronic system. Here, the coherent effects are those in which correlations in phase are maintained between electronic wave functions and optical fields throughout interaction of a light with a material. In order to achieve a coherent interaction, it is necessary that the optical pulse width be shorter than the phase relaxation time of the material in which the interaction takes place. In the case of room temperature bulk GaAs, the phase relaxation time is approximately in the range of 0.1 to 0.2 ps. When the optical pulse width is longer than the phase relaxation time, real carrier generation occurs. The operation speed is restricted by slow longitudinal relaxation due to the real carrier generation, and in addition thereto, the coherent effects are prevented to reveal. It has been shown that even when the optical pulse width is shorter than the phase relaxation time, the real carrier generation stilI occurs via the two-photon absorption process. The higher a pulse repetition frequency, the greater is the accumulation of real carrier generation. Accordingly, though ultra high speed phenomena can be observed with ultra-short pulses in the femtoseconds order, generated by a mode-locked laser, and having a repetition rate of approximately 100 MHz, it is expected that the ultra high speed phenomena cannot be observed due to influences of the real carrier generation as the repetition frequency increases. The above mentioned 100 MHz is a frequency slower than carrier lifetime. For the aforementioned reasons, at the present stage, it is considered to be impossible to establish an all-optical device which is capable of ultra-high repetition operation utilizing resonantly enhanced coherent effects such as AC-Stark effect.
On the other hand, resonantly enhanced incoherent effects caused by real carrier generation is considered to be able to operate at low power smaller than watt, and hence is quite practical. However, switch-off time of a device or relaxation time of nonlinear refractive index is restricted by longitudinal relaxation time or interband recombination time of a carrier. In the case of GaAs, the interband recombination time is on the order of nanoseconds, and thus it is impossible to utilize the high speed of a light.
Accordingly, it is desired to shorten relaxation time of resonant incoherent effects such as band-filling effect by any means. There can be considered several processes for increasing a speed of relaxation time of band-filling effect. One of them is to introduce recombination centers via proton bombardment, but may entail reduction of nonlinearity. The increase in speed of relaxation time due to surface levels is effective to a simple form such as etalon, but may entail doubts in compatibility with waveguide devices and also may have a limitation in increasing the operation speed. By any conventional processes such as above mentioned, it is impossible to achieve operation on the order of picoseconds for taking advantage of the high speed of a light, and in addition, conventional processes entail the reduction of nonlinearity.